The invention refers to a blood-taking device comprising a tubule in which a separating member is disposed which has a specific gravity that is in between the specific gravities of the blood components to be separated.
Such blood-taking devices are known from German Patents 27 11 336 and 27 34 720, as well as from European Patent 0 056 609. In as far as these blood-taking devices use a tubule of comparatively soft plastics material that, during centrifugation, is expanded by the force of the liquid contained therein and causes a gap to occur around the separating member, the softness of the tubule is often considered disadvantageous. In order to be able to use a soft tubule and yet make the blood-taking device more resistant to exterior influences, European Patent 0 305 459 describes a blood-taking device with a comparatively rigid outer tubule and a comparatively soft inner tubule contained therein which may expand during centrifugation. There is an annular gap between the outer tubule and the inner tubule and the inner tubule contains a separating member provided with a breakable plunger rod so that the separating member can also be used as a plunger for drawing a blood sample into the blood-taking device. In this known blood-taking device, the inner tubule is fixed with respect to the outer tubule either by a stopper that closes both tubules at the same time, or by a press fit engagement extending over a larger portion of the length in the front portion of both tubules, there being no annular gap in this portion because of the press fit. Using an elastomeric stopper for the mutual fixation of the two tubules, a sufficiently firm fit cannot be obtained. It is not possible to achieve a sealing or a sufficient fixation between the soft elastomeric stopper and the soft inner tubule. With the other solution in which there is no annular gap in the front portion and the outer tubule is provided with an inner enlargement in this area, the separating member may get into the portion where no annular gap is provided. In this case, the separating member gets stuck and cannot fulfill its function. It is a further problem that during centrifugation leaks may cause liquid to get past the press fit area into the annular gap. If there is a liquid present in the annular gap, the function of the inner tubule, i.e. the expanding during centrifugation, can no longer be fulfilled.
It is an object of the present invention to provide a blood-taking device comprising an inner tubule and an outer tubule, which is very safe to handle and highly reliable and in which tightness during centrifugation is ensured.